Al–Cu intermetallic coatings processed by sequential metalorganic chemical vapour deposition and post-deposition annealing

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This is an author-deposited version published in:

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Eprints ID: 9056

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10.1016/j.apsusc.2012.03.053

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http://dx.doi.org/10.1016/j.apsusc.2012.03.053

To cite this version:

Aloui, Lyacine and Duguet, Thomas and Haidara,

Fanta and Record, Marie-Christine and Samélor, Diane and Senocq,

François and Mangelinck, Dominique and Vahlas, Constantin Al–Cu

intermetallic coatings processed by sequential metalorganic chemical

vapour deposition and post-deposition annealing. (2012) Applied Surface

Science, vol. 258 (n° 17). pp. 6425-6430. ISSN 0169-4332!

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Al–Cu

intermetallic

coatings

processed

by

sequential

metalorganic

chemical

vapour

deposition

and

post-deposition

annealing

Lyacine

Aloui

a

_,

_Thomas

_Duguet

a,∗

,

Fanta

Haidara

b

,

Marie-Christine

Record

b

,

Diane

Samélor

a

,

Franc¸

ois

Senocq

a

_,

_Dominique

_Mangelinck

b

_,

_Constantin

_Vahlas

a

a_CIRIMAT,_Université_de_Toulouse_-_CNRS,₄_allée_Emile_Monso,_BP-44362,₃₁₄₃₂_Toulouse_Cedex_4,_France b_IM2NP,_Université_Paul_Cézanne_-_CNRS,_Faculté_de_St_Jérôme,_Case_142,₁₃₃₉₇_Marseille_Cedex_20,_France

Keywords: Aluminides Intermetallics

Chemicalvapourdeposition Coatings

Annealing

a

b

s

t

r

a

c

t

Sequentialprocessingofaluminumandcopperfollowedbyreactivediffusionannealingisusedasa paradigmforthemetalorganicchemicalvapourdeposition(MOCVD)ofcoatingscontainingintermetallic alloys.DimethylethylaminealaneandcopperN,N′_{-di-isopropylacetamidinate}_are_used_as_aluminum_and

copperprecursors,respectively.Depositionisperformedonsteelandsilicasubstratesat1.33kPaand 493–513K.DifferentoverallcompositionsintheentirerangeoftheAl–Cuphasediagramareobtained byvaryingtherelativethicknessofthetwoelementallayerswhilemaintainingtheoverallthicknessof thecoatingcloseto1mm.As-depositedfilmspresentaroughmorphologyattributedtothedifficulty ofcoppertonucleateonaluminum.Post-depositionannealingismonitoredbyinsituX-raydiffraction, andallowssmootheningthemicrostructureandidentifyingconditionsleadingtoseveralAl–Cuphases. OurresultsestablishaproofofprinciplefollowingwhichMOCVDofmetallicalloysisfeasible,andare expectedtoextendthematerialspoolfornumerousapplications,withinnovativethinfilmprocessing on,andsurfacepropertiesofcomplexinshapeparts.

1. Introduction

Al–Cuintermetalliccompounds present attractiveproperties in applicationssuchas interconnects for integrated circuits [1]

orcorrosionresistantcoatings[2]. Al–Al2Cucompositesfeature

enhancedYoung’smodulus,goodcompressivestrengthand rea-sonablygoodcompressiveductility[3].Inaddition,AlandCuare reasonablyinexpensive,andeasilyavailable.

ProcessingofAl–Cuand,moregenerallyofintermetallicalloy coatingsby metalorganic chemical vapourdeposition (MOCVD) isexpectedtoextendtheirimplementationinsurface engineer-ing. Especially, thanks to the possibility to operate in surface reactioncontrolledregime,MOCVDallows surfacetreatmentof complex-in-shapeitemssuchasglassmoulds,turbinebladesand vanesinaeronauticindustries,orporouspreformswhoseinternal surfacemaybefunctionalizedforthepreparationofsupported cat-alysts.Versatility,costeffectiveness,environmentalcompatibility, andthepossibilitytoprocessfilmscontainingthermodynamically metastablephases,areadditionaladvantagesofMOCVDprocesses. Finally,theuseofmolecularprecursorsallowsoperatingatlowto

∗ Correspondingauthor.Tel.:+33534323439,fax:+33534323498. E-mailaddress:thomas.duguet@ensiacet.fr(T.Duguet).

moderatetemperatures,thusextendingthetargetedapplications spectrumsoastocovertemperature-sensitivesubstrates.

Thepricetopayfor thishighpotentialis theneedtotackle the challenges imposed by thecomplex gas phaseand surface chemistries.Inadditiontomasteringthedepositionreaction,these challenges also concernthe designof the precursors upstream theMOCVDprocess,theengineeringoftheMOCVDapparatusin termsofprecursorvapourgeneration,energydeliverymeans,and dynamicalinsituandonlinediagnosticstomonitorgasand sur-facereactions.Theinherentdifficultyfor theestablishmentofa robustMOCVDprocessisfurtheramplifiedinthecaseofcoatings containingseveralelementsandpotentiallyintermetallicphases, mainlybecauseofthelimitedwidthoftheirstabilitydomains,and thefar-from-equilibriuminitialstatewhichcanleadtounpredicted transitions[4].Moreover,suchaprocessforthepreparationof mul-timetalliccoatingsmustinvolvetheuseofcompatibleprecursors forthedepositedelements.Thegeneralcriteriaqualifyingan inor-ganicormolecularcompoundasprecursorforCVDprocesseswere discussedbyMauryetal.[5,6].InthecaseoftheMOCVDof inter-metalliccompoundsthereareadditionalonessuchas(a)similar transport behaviours,(b)absenceof heteroatomsin theligands whichmayreactwiththeothermetal,(c)compatible decompo-sitionschemes,andifpossible(d)belongingtoacommonfamily ofcompounds.Untilnow,thissituationresultedinlimited investi-gationofMOCVDfortheco-depositionofintermetallicalloyfilms.

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ReportsontheMOCVDofAl–Cuwerepublishedin thenineties involvingaluminumalaneandcopperphosphineprecursors[7,8]. However,theyconcernfilmswithlowcopperconcentration,atthe levelof1wt.%,targetingthedopingofAl-basedinterconnectionsin microelectronicsratherthantheprocessingofcoatingscontaining intermetallicphases.

Theabovementionedconstraintscanbepartiallycircumvented ifthepreparationofthecoatingproceedsintwosteps.Namely, sequentialdepositionoftheelementsintheformofbi-or multi-layers,isfollowedbyanappropriatelytunedannealingwhichleads totheformationofthetargetedphases.Thislattersolution,applied intheprocessingofAl/Cubilayers,isadoptedinthepresentstudy. Optimizationofthedepositionreactor,processingoftheAlandCu unaryfilms,investigationoftheirdecompositionmechanisms,and kineticmodellingoftheMOCVDprocessarepresentedelsewhere ([9,10]andreferencestherein).

The article is presented asfollows. Theexperimental proto-colinvolvingMOCVDofAlandCu,andpost-depositionannealing ispresentedin details,first.Then,themicrostructureand com-positionprofilesoftheas-processedAl-richandCu-richbilayers arecompared.Finally,microstructureandphasetransitionsofthe annealedcoatingsarepresentedanddiscussed,priortoproviding concludingremarks.

2. Materialandmethods

Depositionsareperformedintheexperimentalsetupdescribed indetailsandmodeledinRef. [11].Thesetupiscomposedofa stagnantflow,cylindrical,stainless steelreactor.Thedeposition chamberfeaturesadoubleenvelopeallowingthemonitoringof wallstemperaturethroughthecirculationofthermallyregulated silicon oil. A turbomolecular pump ensures a basepressure of 1.3×10−4_Pa._The_pumping_group_is_protected_from_the_corrosive

by-productsbyaliquidnitrogentrap.Gasisdistributedthrough ashowerheadsystem,describedandmodeledinRef.[12].Gases arefedthroughelectropolishedstainlesssteelgaslineswithVCR fittingsandtheirflowrateiscontrolledbycomputerdrivenmass flowcontrollers.

5mm×10mm up to 20mm×20mm 304L stainless steel couponsareusedasrepresentativeoftechnologicallyinteresting substrates.Thermallyoxidizedsilicon(140nmSiO2)couponsare

usedfortheeaseofcrosssectionspreparationforobservationby scanningelectronmicroscopy(SEM),andforX-raydiffraction anal-yses(XRD).Substratesareplacedhorizontallyona58mmdiameter susceptorstandingbelowtheshowerhead.Theyareheatedbya resistancecoilgyredjustbelowthesurfaceofthesusceptor. Stain-lesssteelsubstratesarepolisheddownto4000SiCpapergrade andaresonicatedinacetoneandanhydrousethanol.Siliconwafers aredegreasedina70%H2SO4–30%H2O2solution,rinsedwith

de-ionizedwateranddriedunderargonstreambeforeuse.Asetof fivestainlesssteelandfiveSiO2substratesisusedineachrun.

Sub-stratesareexposedtoatmosphereforalimitedtimeduringtheir transferbetweenthepreparationlabandtheirloadinginto vac-uum.Priortodeposition,insituradiofrequency(RF)Ar–10%H2

plasmaetchingisappliedwithinputpower40Wat120kHz,in conditions160Paand493K,for30minwiththeaimtorecoveran organic-pollution-freesteelorsilicasurface.Inallexperimentsthe operatingpressureandthetemperatureofthereactorwallsare fixedat1.33kPaand368K,respectively.

Adduct gradeDMEAA(SAFC Hitech)is usedas-receivedina stainlesssteel bubbler. It is maintained at 281K by immersion in a thermoregulated water bath. The corresponding saturated vapourpressure is 99Pa. TheDMEAA bubbler is maintainedat thistemperatureduringtheentireperiodofitsserviceinorder to avoid degradation of the precursor [13]. 25 standard cubic

centimetres(sccm)of99.9992%purenitrogen(AirProducts) bub-blesthroughtheAlprecursor.Assumingsaturationofthegasphase, theseconditionsleadtoanupperlimitoftheDMEAAflow rate equalto2sccm[14].Thetotalflowrateiscompletedto327sccm byadding300sccmofN2asadilutiongas.Deposition

tempera-tureisfixedat493K.Ithasbeenpreviouslyshownthatinthese conditionsa meannucleationdelayof7minprecedes initiation ofthegrowthofAl.Nucleationdelayisdeterminedby observa-tionof changeofthecolour of thesurfaceand is thesamefor SiO2 and stainlesssteelsamples.Algrowthrateismeasuredat

certainpositionsoverthesusceptor,throughtheweightgainof eachsample[10].Weightgainispreferredtothickness measure-mentsincefilmporosityandroughnessinduceanoverestimation ofthegrowthrate.Forinstance,SEManalysisofthecross-section ofanAl(resp.Cu)sampleshoweda1300nm(resp.110nm)thick film,whereasthethicknessdeterminedbyweightmeasurement was 400nm (resp. 45nm), only. Al growth rate is mapped as beingconstantat12.6mmolcm−2_h−1 _on_the_central_part_of _the

susceptorandgraduallyincreasingbeyondaradiusof15mmto reach15.6mmolcm−2_h−1_at_the_edge_of_the_susceptor,_averaging

13.3mmolcm−2_h−1_on_the_entire_heated_surface.

[Cu(i-Pr-Me-AMD)]2 (NanoMePS, www.nanomeps.fr, last

accessedOctober 7,2011)isusedas-receivedforCudeposition. ThisprecursorisappropriateforuseinaprocessinvolvingCVDof AlfromDMEAAbecause(a)thetwoprocessingconditions win-dowspartiallyoverlap,(b)itcontainsneitheroxygennorhalogens intheligands,noritrequiresoxygencontainingco-reactantsfor thedepositionofcopper[9].[Cu(i-Pr-Me-AMD)]2ismanipulated

in glove box and is conditioned in a packed bed loaded in a homemadesublimatorcomposedofafullstainlesssteelbody,a fritandVCRfittings.Aloadof500mgoffreshcompoundisused in each run. During deposition the precursor is maintained at 368K withthermally regulatedheatingtapes,this temperature correspondingtoasaturatedvapourpressureof36Pa[9].50sccm of N2 are fed through the copper precursor corresponding to

anupperlimit oftheflow rateof[Cu(i-Pr-Me-AMD)]2 equal to

1.2sccm.50sccmof95%purehydrogen(AirProducts)isusedas reducinggas.TherelativelylowpurityofH2doesnotimpactthe

purity ofthe deposited Cu.Similar tothedepositionof Al, the totalflowrateiscompletedto326sccmbyadding225sccmof N2 dilution gas.Cudepositionisperformedat 513K.Themean

growthrateofCuintheseconditionswaspreviouslydetermined byweightgaintobeequalto0.3mmolcm−2_h−1_[15]_.

Thedepositionprotocolconsistsin(a)establishingflowratesin allthegaslines,bypassingtheprecursorvessels,(b)establishingthe targetedtemperatureateachpartofthesetupexceptforthe cop-persublimator,(c)performingthedepositionofAl,(d)bypassing theDMEAAbubblerfor30min,(e)heatingthecoppersublimator to368K(10min)andincreasingthetemperatureofthe suscep-torto513K,(f)performingdepositionofCu,and(g)bypassingthe precursorvesselswhilecoolingdownthesusceptor.This proto-colpresentsthedrawbackofmaintainingthefreesurfaceofthe depositedAlduring40min(steps(d)and(e))priorthedeposition ofCu,runningtheriskofcontaminationoftheAl/Cuinterfaceby residualoxygen.However,contaminationlevelislowerthanthe oneobtainedifusingO-containingCuprecursors.

Severaldepositionrunsareperformedinthesameconditions, thedifferencebeingthedurationsofthedepositionofAl(between 20minand57min)andCu(between170minand960min).

Post-depositionannealingisappliedtotheas-processedAl/Cu bilayersinordertoinvestigatereactivediffusion,andobtain coat-ingscontainingdifferentintermetallicAl–Cuphases.InsituXRD measurements in Bragg–Brentano configuration are performed duringheattreatmentsintwoinstruments,operatingwithCuKa, Nifilteredradiation: aBrukerD8Advance,fittedwitha Vantec SuperSpeeddetectorandaPhilipsX’pert.Theyareequippedwith

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Fig.1.SurfaceSEMmicrographofanAlfilmdepositedonthermallygrownsilicain theadoptedconditionswithinsituplasmapre-treatment.

aMRI(undervacuum)andanAntonPaarHTTK(undercontrolled atmosphere)hightemperaturechambers,respectively.TheX-ray diffractogramsarerecordedfromroomtemperatureupto928K bystepsof30◦_._{Crystallographic}_{characteristics}_of_the_as-deposited

andannealedcoatingsaredeterminedbyXRDingrazingincidence usingaSeifertXRD3000TTinstrument(CuKa,graphitediffracted beammonochromator).

Thearithmeticaverageroughness(Ra)ofas-depositedbilayers

andpostannealedcoatingsisdeterminedwithaZygoMetroPro, NewView100opticalprofilometer.Theirmorphologyisevaluated witha LEO 435-VPscanning electron microscope.The elemen-talcompositionofthecoatingsisdeterminedbyElectronProbe Micro-Analysis(EPMA)withaCAMECASX-50apparatus,equipped withthreewavelengthdispersivespectrometers.Depthprofilesare determinedbyradiofrequencyglow dischargeopticalemission spectrometry(RFGD-OES)withaHoribaScientificGD-Profiler2. RFGD-OESuses alow pressure plasmafor fastsputtering(few mm/min)ofthesurfaceofthesampleandexcitationofthe sput-tered atoms. Light emitted by sputtered atoms during species de-excitationiscollectedbyapolychromatorthatisabletodetect allelementsfarfromUVtolowinfrared(includingH,O,CorN species).Theuseofradiofrequencysuppressestheconstraintof usingconductingsamples,henceoxidizedSisubstratescanbe char-acterizedreadily.

3. Resultsanddiscussion 3.1. As-depositedfilms

As-depositedfilmsaresystematicallycomposedofelementalAl andCu.Accordingtodepthprofilingresults,reactivediffusionatthe Al–CuinterfaceisinitiatedduringthedepositionofCu(deposition at513K),butthequantityofintermetallicphasesisnotenoughto revealcorrespondingpeaksintheX-raydiffractograms.The con-tentofheteroatomsduetotheprecursorligandsortotheresidual atmosphereofthereactorvessel(oxygen,nitrogen,carbon)is sys-tematicallybelowthedetectionlimitofEPMA(<1at.%),withour apparatus.

Fig.1presentsaSEMmicrographofAldepositedonthermally grownsilicaafterinsitu plasmapre-treatmentofthesubstrate (Fig.1a)plasmapre-treatmentyieldingasmoothersurface.Ravalue

forthisfilmis0.024mm,tobecomparedwith0.105mmfor unpro-cessedsubstrates.

CudepositiononAlfilmsprovidesbilayerswhosesurface char-acteristicsdependonthequantityperunitsurfaceofthedeposited Cu.SmallquantitiesofdepositedCu,correspondingtoahighAl:Cu

Fig.2.Al-richAl/Cubilayerwithoverallcomposition11at.%Cu.(a)Surface(bottom) andcrosssection(top)SEMmicrographsand(b)RFGD-OESdepthprofileofabilayer formedonsilica.

ratio,do notmarkedlymodifythemorphologyofthefilmwith regardtotheoneshowninFig.1.ASEMcrosssectionanda sur-faceviewofanas-depositedbilayerwithanoverallcomposition of11at.%CuisshowninFig.2a.Thecrosssectionofthefilmis typicalofthatofCVDAl[16].Raofthefilmequals0.087mmand

thesurfacemicrostructureisorganizedintwolevels:anAllayerof approx.1mm,andathinnerCulayerof0.1mm.

Abetterinsightintheorganizationofthebilayerisobtainedby performingtheRFGD-OESdepthprofileanalysis.Fig.2bshowsthe sampleelementalcompositionvs.depth.Fourlayerscanbe iden-tified.Startingfromthesurface,wedeterminethattheCulayer is0.08mm-thick,andthatitcontainsCandO.Cusitsontopofa 0.75mm-thickAllayerthatalsocontainsCandOcontaminants.Itis interestingtonotethatCudiffusesdeepbelowthesurface(approx. downto0.8mm).AsmallCupeakbetween0.65and0.80mmdepth maycorrespondto fastdiffusionof Cuin preferential diffusion pathsand Cusilicidesformation,ortotheabruptchangeofthe sputteringrateattheAl/silicainterface.

CandOpeakspresentattheAl/Cuinterfacemaycorrespond toaslightoxidationoftheAlsurfacebeforeCudepositionstarts. Thethirdlayer(between0.8and1.0mmdepth)correspondstothe thermallyoxidizedSiOxlayer,followedbythepureSiwafer(depth

>1.4mm).

TopographyofoursampleshastobeconsideredforRF GD-OES analysesbecausesputtering occurson alargesurface area (2mmdiameteranode)[17].First,samplespresentarough mor-phologywithmoreorlessdenseregions,porosities,andsuch.This

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Fig.3. Cu-richAl/Cubilayerwithoverallcomposition90at.%Cu.(a)Surface (bot-tom)andcrosssection(top)SEMmicrographsofanas-processedsamplegrownon silica.(b)GD-OESdepthprofileofabilayerformedonstainlesssteel.

canresultinthesimultaneoussputteringofthesurfaceandofa deeperfeature,suchasanopenporosity.Therefore,itis impor-tanttocarefullyevaluatecontaminantslevelandsharpinterfaces composition.Withflat,dense,andsmoothsamples,aflat-bottom crateriscreatedwithRFGD-OESthatcorrespondstoa pseudo-layer-by-layeretching.Withimperfectsampleswesuspectthata deeper(next)layermaybesputteredbeforetheendoftheprevious layersputtering.Intheprofileplot,thiswouldcorrespondto visu-alizingtwo(ormore)elementscomingfromdifferentlayersatthe sametimeevenwhereasthechemicalinterfaceisactuallysharp, normaltothesurface.Anotherpointistheabsenceofultra-high vacuumthatmightimplythatO,H,andCcompositionsarelargely overestimatedcomparedtorealvaluesinthefilms.EPMA mea-surementsafterAlandCudepositionsshowcontaminantslevels undertheapparatusdetectionlimit(<1at.%).Additionally, hydro-genisknowntohaveasubstantialeffectontheintensitiesofthe opticalemissionspectra[18].Weignoredthiseffect,anddonot presenthydrogenin thequantitativeprofiles,althoughHsignal wasacquired.Sincethesignalsarenormalizedto100%,neglecting hydrogeninducesanotheruncertaintyontheoverallcomposition. Large quantities of deposited Cu, corresponding to Al–Cu coatingswithhighCu content yieldas-deposited bilayerswith roughsurface.Elementalcompositionofthefollowingsamplesis 90–92at.%Cu,dependingontheirrelativepositionintheMOCVD reactor.Fig.3apresentsacrosssectionandsurfaceSEM micro-graphsofanas-processedfilmonthermallygrownsilica.Thecross sectionrevealsaporousmicrostructurewhichwasdevelopedon athinsublayerofAl.Thisisalsoillustratedonthesurfaceview

ofthesamplewhichpresentsamicrostructurewithRaequal to

0.161mm,typicaltothatofCVDCu[15].Densityfunctional the-ory(DFT)calculationsrevealedthatthefirststageofnucleationof CuonatomicallysmoothsurfaceofAlisnotenergeticallyfavoured. AdsorbedCuadatomsarethusexpectedeithertosegregatebeneath thesurfaceofAlortoformnucleusonsitesofhighenergy,such asothersurfaceadatomsordefects[19].WhennucleationofCu isachieved,additionalCuadatomsareexpectedtodiffuseonthe surfacesoastocontributetothegrowthoftheexistingCugrains. Althoughnottakingintoaccounttheproperreactionschemeofthe MOCVDprocessandtheinitialsurfacestate,thisgrowthmodeis atleastpartiallyresponsibleforthefinalroughmicrostructureof thefilm;i.e.throughtheformationofadiscontinuousfilmwith importantopenporosity.

TheGD-OESelemental profileof anAl/Cubilayerformedon stainlesssteelisshowninFig.3b.Weidentifydifferentlayersfrom thisplot.Theextremesurfacelayer(approx.20nm)correspondsto adsorbedCandOspecies.Then,aCulayerextendsover0.25mm.It seemstocontain10at.%O.Consideringthecommentsmadeabove andbecauseEPMAdoesnotdetectO,weassumethatthe effec-tiveOcontentislower.Thislayerisfollowedbyaslightlyoxidized Al/Cuinterface,asshownbytheOpeakat0.25mm.Thefollowing 0.1mm-thickAllayershowsaveryhighcontentofCuthatconfirms thefastdiffusionofCuatdepositiontemperature(513K).Wedo notknowwhere/howdiffusionoccurs,though.ThefactthatX-ray diffractionresultsshowfcc-Alatlowtemperaturesimpliesthatat mostan(Al)solidsolutionisformed,butobviouslynotwith59at.% Cuasitseemstobeat0.3mmdepth.Again,excitedCu*andAl* atomsco-existintheGDplasmabecausetheyaresputteredatthe sametime,butnotnecessarilybecausetheyco-existintheAllayer. Finally,theinterfacewithstainlesssteeliscomplex,withpossible interdiffusionofAlandCuintothesubstrateandofFeandalloying elementsintothecoating.

3.2. Post-depositionannealing

Fig. 4 presents two high temperature X-ray diffractograms obtainedforcoatingscontaining19at.%Cu(a)and35at.%Cu(b). Thethicknessofbothsamplesisapprox.1mmafterannealing,as determinedbySEMcrosssectionanalyses.Theirmicrostructureis compactandhomogeneous(nolayersaredistinguishable)showing thatthewholecoatingthicknessisalloyed.XRDintensitylevelsare representedbythegrayscale.Inbothsamples,reactionbetween AlandCuduringheattreatmentresultsintheevolutionoftheAl andCupeaks,andtheappearanceofpeakscorrespondingto inter-metalliccompounds.PeaksofAlandCueitherdisappearorare shifted(Fig.4a).Afterheattreatment,sample(a)iscomposedofa mixtureofsolidsolutiona-(Al),u-Al2Cu,andasmallamountofCu

forwhichapeakisstillvisible.Sample(b)issingle-phased,with onlyh-AlCupeaks.Heattreatmentofsample(a)wasstoppedat 853Kbecauseofthevanishingofthediffractionpeaksatthis tem-perature.Thisisattributedtotheappearanceofaliquidphaseas canbeconcludedbytheeutecticreactionoftheAl–Cuphase dia-gramoccurringat821Kforalloyswithcompositionsbetween(Al) andAl2Cu[20].Thesameconclusionisdrawnforsample(b)where

annealingshouldhaveresultedintheformationofatwo-phase samplecomposedofu-Al2Cuandof12%weightfractionofh-AlCu.

ThevanishingoftheAl2Cupeaksabove823K(550◦CinFig.4b)

maycorrespondtothemeltingofAl2Cu,thatshouldoccurabove

863K.

AccordingtotheinsituX-rayanalysis,AlandCufirstreactto formAl2Cu.IfthecompositionisrichenoughinCu(sampleb),then

Al2CureactswiththeremainingCutoformAlCu,above823K.For

sample(b),wesuspectthatg-Al4Cu9isalsoformedalongwith

u-Al2Cu.Itsmaindiffractionpeak(330)appearsataboutthesame

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Fig.4.X-raydiffractogramsrecordedasafunctionoftemperatureduring post-depositionannealingforsampleswithcomposition19at.%Cu(a)and35at.%Cu (b).

HoweverifAl4Cu9isformed,itischaracteristicofatransientphase

becauseit wouldappear anddisappearduringtheformationof Al2CuwhilebothAlandCuarestillpresent[21].

Fig.5 presentsasurface SEMviewofthesamplecontaining 19at.%Cuafterannealingat853K.Thesurfaceappearsasbeing

Fig.5.LowmagnificationSEMsurfaceviewofafilmwithoverallcomposition 19at.%Cu.NotationscorrespondtoatomicpercentofCu.

non-homogeneouswithatleasttwozones,illustratedbycurved brightstripesandagreybackground.ThevalueofRainthiscase

is0.212mm.EPMAanalysisrevealedthepresenceofthreezones withvariableelementalcomposition,namely5–8,17and30at.% Cu.Althoughtheprobesizeoftheinstrumentis5mm;i.e.larger thanthetypicallengthscaleoftheobservedfeatures,theseresults confirmtheheterogeneityofthesample.Thismicrostructureisdue tothethermalhistoryofthefilmwhichwascooleddownfroma partiallyliquidstatetoroomtemperature.Thecompositionofthe threezonesofthefilmfitstheeutecticequilibriumattheAl-rich sideoftheAl–Cuphasediagram,involving3at.%Cusolidsolution fcca-(Al),u-Al2Cuandaliquidphaseat17at.%Cu.Thepresence

inthefilmofaphasewhosecompositioncorrespondstotheliquid phaseisattributedtotherelativelyhighcoolingrateof30◦_/min.

Basedontheseremarks,elementalcompositionswerenotedinthe threephasesofthemicrographinFig.5.Theultimate microstruc-tureofsuchcoatingsiscompact.Itconfirmsthedeterminedoverall compositionofthecoating,andillustratesthecoherencebetween theoverallcompositionofthecoatingandthecorrespondingphase equilibrium,atleastinthisregionofthephasediagram.

4. Conclusions

Bilayers ofCuand Alweredeposited byMOCVDat 1.33kPa and temperatures 493K and 513K on stainless steel and oxidized Si substrates with the aim to process Al–Cu inter-metallic alloy films. Dimethylethylamine alane and copper N,N′_{-di-isopropylacetamidinate} _provided _pure _Al _and _Cu _films,

respectively.Alfilmsaresmooth,whereassubsequentlydeposited Cu yields surfaces whose roughness increases with increasing thickness.Reactivediffusionisobservedthroughpost-deposition annealing,inducingtheformationofAl–Cuintermetallicphases. InsituX-raydiffractionmeasurementsduringheattreatmentallow investigationofthereactionpath.Depositionofdifferentrelative thicknessesofCuandAlfilmsleadtodifferentsample composi-tions,andallowstheexplorationofdifferentphasespacesofthe Al–Cuphasediagram.Filmscontainingu-Al2Cu,h-AlCu,andlikely

g-Al4Cu9areobtained.

Forthcoming publications will focus on the investigation of thesampleswithanintermediateCucontent,andofappropriate annealingconditions,withtheaimofcreatingsingle-phase inter-metalliccoatingsoftechnologicalinterest(g-Al4Cu9 orz-Al3Cu4,

forinstance).

TheobtainedresultsshowthatMOCVDassociatedwith post-deposition heat treatment is a valid way to obtain films of intermetallicalloys,pavingthewaytoconformaldepositionofthis typeofmaterialsfornumerousapplicationfields.

Acknowledgements

We are indebted to Sophie Gouy and Philippe de Parseval, ObservatoireMidi-Pyrénées,Toulouse,andtoCéliaOlivero,Horiba Jobin-YvonSAS,forEPMAandRF-GD-OESanalyses,respectively. Thisworkwassupportedbythe6thFrameworkEUNetworkof Excellence‘ComplexMetallicAlloys’(contractno. NMP3-CT-2005-500140),andbytheFrenchAgenceNationaledelaRecherche(ANR) undercontractno.NT05-341834.

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